As exhaust emission control catalysts for efficiently purifying carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and the like that are discharged from internal combustion engines, three-way catalysts have been in widespread use. As a representative configuration of a three-way catalyst, it is possible to mention a configuration in which one or a plurality of catalytic metals that are selected from platinum (Pt), palladium (Pd), rhodium (Rh) and the like as platinum group elements are supported on a surface of a porous carrier that is made of oxides having high heat resistance.
In each of the aforementioned exhaust emission control catalyst, there is sort of good or bad affinity between the used catalytic metals and the porous carrier, and there is a suitable combination depending on various objects such as an improvement in catalytic performance, the enhancement of manufacturing efficiency and the like. For example, conventionally, studies have been conducted on the composition and the like of a porous carrier capable of effectively suppressing the sintering (grain growth) under a high-temperature condition of a certain catalytic metal in the case where the catalytic metal is adopted. Besides, attempts have been made to define the range of a suitable combination by focusing attention on the electronegativity related to the aforementioned catalytic metal elements or elements constituting the aforementioned carrier as an index for systematically controlling and understanding a suitable combination of catalytic metals and a carrier composition as described above (e.g., Patent Documents 1 to 3).
By the way, in the case where two or more catalytic metals are used as the aforementioned catalytic metals, the catalytic activity may deteriorate due to a decrease in specific surface area resulting from the sintering of the catalytic metals or due to the alloying of a plurality of catalytic metals or the like, in the process of using the catalyst under a high-temperature condition (e.g., 800 to 1000° C.). Thus, with a view to suppressing the occurrence of these phenomena, it has been proposed to spatially separate the aforementioned two or more catalytic metals from one another, have them supported on different carriers respectively, and realize a multilayer-type catalyst configuration (typically a double-layer-type laminate structure). For example, in Patent Document 4, there is disclosed an exhaust emission control catalyst with a first catalytic layer that has platinum (Pt) supported on a surface of a substrate, and a second catalytic layer that has rhodium (Rh) supported on the first catalytic layer.